This set of Engineering Materials & Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Corrosion and Oxidation Control”.
1. Which of these materials is not added to iron for improving its oxidation resistance?
Explanation: Alloying elements in sufficient quantities are added to iron to improve their oxidation resistance. Nickel, chromium, and aluminum are the most common of these alloying elements.
2. How much chromium is required to provide oxidation resistance for use over 1000oC?
Explanation: Chromium is added to materials to improve oxidation resistance. Up to 10% chromium is added to oil refinery parts and up to 12% is added for operations up to 1000oC. Over this temperature, 17% chromium is added.
3. Molybdenum with hydrogen can be used up to ________
Explanation: Molybdenum in a protective coating of hydrogen can be used for oxidation resistance. It is mainly used in furnace windings up to 1500oC.
4. 10% chromium mixed with ______ can be used up to 1100oC.
Explanation: 10% chromium alloyed with nickel results in an element known as chromel. This element, along with alumel (2% Al, 2% Mn, 1% Si, and Nickel) can be used up to a temperature of 1100oC.
5. Which of these metals does not require alloying for oxidation resistance?
Explanation: Molybdenum is generally covered with a protective oxide coating while titanium acts as one. As a result, no alloying elements are used to improve their oxidation properties, unlike elements like copper and molybdenum.
6. _________ is used to prevent duralumin from corrosion.
Explanation: Alclad is a substance used to prevent corrosion of duralumin. It is made by covering each side of duralumin with a layer of pure aluminum sheets.
7. Galvanized ion is made by dipping low carbon steel in molten zinc at ________
Explanation: Galvanized ion (G.I.) is prepared by dipping a low carbon steel sheet in a molten bath of zinc at around 450oC. When cooled, the zinc coating crystallizes and forms ‘zinc flowers’.
8. Which of these is not a way to prevent intergranular corrosion?
a) Reduction of carbon level
c) Addition of carbide forming elements
d) Reduction of alloying elements
Explanation: Improper heat treatment of 18/8 stainless steel may result in intergranular corrosion. It can be prevented by the shrinking of carbon level to less than 0.05%, quenching from high temperature, and the addition of strong carbide forming elements to the steel.
9. Dezincification of brass is prevented by addition of ________
Explanation: Dezincification is a special type of corrosion that occurs in alpha brass. It can be controlled by adding 0.04% of arsenic to brass.
10. Season-cracking can be prevented by cold-working of brass at _________
Explanation: Cold-worked brass with high residual stresses along the grain boundary is known as season-cracking. This generally occurs in the presence of ammonia. This corrosion can be prevented by cold-working at 300oC.
11. How can caustic embrittlement be avoided?
a) Addition of nickel
b) Increasing potential
c) Decreasing carbon
d) Adding tannins
Explanation: Caustic embrittlement is caused in boilers due to the stress and high concentration of hydroxyl ions. This can be prevented by the addition of tannins and phosphates.
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